1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device for displaying video information by arranging scan signal lines and information signal lines in a matrix and driving them by applying a scan signal and an information signal, respectively, with a memory property in the liquid crystal.
2. Related Background Art
A refresh scan type CRT has been primarily used as a computer terminal display device with a frame frequency of no lower than 60 Hz being used to prevent flicker and a non-interlace system has been adopted to improve visibility of a moving display of screen information (the movement of a mouse or an icon). (Note: In a television system, the interlace system has been adopted to display animation, and a field frequency is 60 Hz and a frame frequency is 30 Hz). Thus, the higher a display resolution is, the higher is the power required, and a drive control unit is larger in size and more expensive.
Recently, a flat panel display has been attracting the notice because of the inconvenience of the CRT due to its large volume and high power consumption.
Several flat panel display systems are known. For example, a high time-division system of twisted nematic liquid crystal (STN), a monochromatic version thereof (NTN) and a plasma display system adopt the same image data transfer system as that of a CRT and adopt the non-interlace system with a frame frequency of no lower than 60 Hz as an image refresh system. This is because these display panels have no memory property in principle and hence require a refresh cycle of no lower than 60 Hz in the frame frequency in order to prevent flicker. In a system in which switching transistors or non-linear devices are built in respective pixels of the twisted nematic liquid crystal (TFT, MIM, TFD etc.), image information can be maintained for one frame period at most. Thus, this system also requires a refresh cycle of no lower than 60 Hz.
On the other hand, a ferroelectric liquid crystal display device has a property of persistence of image information which has once been displayed (memory property) and hence it can display the image information on a much larger screen than those of the above-mentioned display devices with a higher resolution. However, because it uses a low frame frequency drive system, a partial rewriting scan system (in which only those scan lines whose image information has changed are scanned (driven)) which makes use of the memory property is necessary. The partial rewriting scan system is disclosed in U.S. Pat. No. 4,655,561 to Kambe et al and U.S. Pat. No. 5,091,723 to Konno et al, and a low frame frequency refresh drive system using multi-interlaced scan is disclosed in U.S. Pat. No. 5,058,994 to Mihara et al.
In the prior art control method for a ferroelectric liquid crystal display device, the partial rewriting scan is conducted when the image information has changed, and the total screen refreshing scan by the multi-interlaced scan is conducted when the image information does not change. Thus, even if there is no key entry or mouse scan because of the absence of an operator, the display panel continues the refreshing scan by the multi-interlaced scan. As a result, if the same image is continuously displayed on the screen for a long period because of no manipulation, the display screen is burnt and drive margin is lowered, which causes the degradation of image quality.